A lambda closed-loop control in conjunction with a catalytic converter is currently the most effective exhaust-gas treatment method for the spark-ignition engine. Only in interaction with currently available ignition and injection systems is it possible to achieve very low emission values. Especially effective is the use of a three-way catalytic converter, or selective catalytic converter. This type of catalytic converter is able to break down more than 98% of hydrocarbons, carbon monoxides and nitrogen oxides provided the engine is operated within a range of approximately 1% around the stoichiometric air-fuel ratio at lambda=1. In this context, lambda specifies the degree to which the actually present air-fuel mixture deviates from the value lambda=1, which corresponds to a mass ratio of 14.7 kg air to 1 kg of gasoline theoretically necessary for complete combustion, i.e., lambda is the quotient of the supplied air mass and the theoretical air requirement.
In lambda control, the given exhaust gas is measured and the supplied fuel quantity immediately corrected in accordance with the measuring result with the aid of the injection system, for instance. Used as sensor is a lambda sensor, which has a voltage jump at precisely lambda=1, in this way supplying a signal that indicates whether the mixture is richer or leaner than lambda=1. The effect of the lambda sensor is based on the principle of a galvanic oxygen-concentration cell with a solid state electrolyte.
Lambda sensors designed as two-step sensors operate in a manner known per se according to the Nernst principle, on the basis of a Nernst cell. The solid state electrolyte is made up of two boundary surfaces separated by a ceramic. The ceramic material that is utilized becomes conductive to oxygen ions at approximately 350 degrees Celsius, so that, given different oxygen concentrations on both sides of the ceramic, the so-called Nernst voltage is then generated between the boundary surfaces. This electric voltage is a measure for the difference in the oxygen concentrations on both sides of the ceramic. Since the amount of the residual oxygen in the exhaust gas of an internal combustion engine largely depends on the air-fuel ratio of the mixture conducted to the engine, the oxygen concentration in the exhaust gas may be utilized as a measure for the actually present air-fuel ratio.
In a rich mixture (Lambda<1), the sensor voltage supplied by the lambda sensor in accordance with the oxygen concentration in the exhaust gas reaches 800 to 1000 mV; in a lean mixture (Lambda>1), approximately 100 mV are still reached. The transition from rich to lean range occurs at around 450 to 500 mV. A stoichiometric ratio of air to fuel (Lambda=1) results in a sensor voltage of 450 mV. The mentioned values hold true for an operating temperature of the ceramic body of approximately 600 degrees Celsius, so that it must be heated accordingly during operation of the Lambda sensor.
The mentioned stepped voltage characteristic of the previously described lambda sensors allows a regulation in only a narrow value range around lambda=1. Therefore, these sensors are also called lambda=1 jump sensors. A substantial broadening of this measuring range, to lambda between 0.7 and 4, may be achieved with so-called broadband lambda sensors (FIG. 1) in which additionally to the Nernst cell a second electrochemical cell, the so-called pump cell, is integrated. As described in the figurative part that follows, a voltage present at the pump cell is regulated such that a status of lambda=1 is constantly maintained in a cavity or measuring gap. The electric pump current induced in the process is proportional to the oxygen concentration, or, in values below zero, proportional to the O2-requirement in accordance with the inflowing fuel concentration, and therefore a measure for the value of lambda in the exhaust gas.
Furthermore, in the lambda=1 jump sensors described initially, it is known to maintain the sensor voltage at 450 mV in the start-up phase of the engine by means of a pilot control, via a voltage divider, for as long as the sensor is still too cold and no Nernst voltage corresponding to the oxygen concentration is present as sensor output signal. For in the cold state the sensor still has high internal resistance.
In contrast, the known broadband lambda sensors are operated using an evaluation circuit, which compares the Nernst voltage with an internally generated voltage of 450 mV. As soon as a deviation has occurred, it is increased in the circuit and injected into the pump cell as pump current. In this way, oxygen is pumped into or out of the cavity, and the Nernst voltage stabilizes at 450 mV. It is thus disadvantageous in these sensors that, during the start-up of the engine or the sensor, the Nernst voltage increases only slowly from 0V to 450 mV even if the exhaust gas is at lambda=1 the entire time. Because of this deviation, the amplifier applies the full positive pump voltage to the pump cell. Only after sufficient heating of the sensor will a high pump current set in that empties the cavity, despite the fact that the correct gas concentration lambda 32 1 had been present from the beginning. Thus, the output signal exhibits an overswinger in the lean direction, which seriously interferes with the regulation. Subsequently, the oxygen must be replenished again; in doing so, a small overswinger in the rich direction occurs again.